Different kinds of structure in aerogels: relationships with the mechanical properties

Abstract

The density and the structure (fractal and non-fractal) of aerogels are modified either by the adjustment of the gelifying concentration, by a precise control of the viscous flow sintering process or by an isostatic pressure deformation. These aerogels have porosities ranging from 98% to 0%. The mechanical properties of the different aerogels (elastic modulus and strength) measured by 3 point bending, are dependent on their structure; they vary by five orders of magnitude as a function of density and follow power law evolution. However for the same relative density the elastic modulus and strength can increase by one order of magnitude due to a change in connectivity. These structural differences have been observed by SAXS experiments. The effects of the sintering process compared to that of the plastic transformation on the mechanical properties are explained by the associated structural changes. Sintering increases the network connectivity and the densification by compression leads to a new spatial arrangement of the clusters but their internal structure is not affected. In addition, relationships between structural and porous features and the mechanical properties are discussed in terms of percolation theory and the fractal approach. We show that the exponent of the power law does not depend on the fractal feature and percolation is only an approximation which cannot describe results.